It is known in the art relating to vehicle vacuum supply systems to utilize the intake system of an internal combustion engine as a source of vacuum for driving various accessory devices; such as, for example, vacuum brake boosters, heating and ventilating system motors, speed or cruise control devices and various other accessory devices and motors. It is also known that the intake vacuum of throttle controlled spark ignition engines, for example, varies significantly under differing operating conditions from a relatively high degree of vacuum under highly throttled, low speed and load operation to relatively small vacuums under wide open throttle conditions of heavy engine load.
In the past, when the use of vacuum actuated windshield wipers was common, the problem of the wipers stopping during vehicle acceleration due to reduced engine vacuum was overcome in some cases by providing a booster vacuum pump in series with the intake manifold vacuum supply connection. Such a pump was sometimes provided as an adjunct to the engine-driven fuel pump to provide sufficient vacuum supply for maintaining operation of the wipers under conditions of reduced engine vacuum.
In recent years, vacuum-powered windshield wipers have generally been replaced by electric motor-driven devices, while vacuum power has been utilized increasingly for the operation of vacuum assisted power brakes as well as various other accessory devices. In such systems, the problem of temporarily reduced engine vacuum, such as during acceleration, has been taken care of by provision of a large storage reservoir for the vacuum brake booster so that the use of supplemental or booster vacuum pumps has essentially disappeared.
Today, demands for ever increasing vehicle fuel economy are leading to the use of smaller sized engines and lower engine cruising speeds resulting in engine operation during highway cruising at throttle positions much closer to the high economy, wide open throttle condition than was previously the case. As a result, the level of vacuum produced by a vehicle engine under highway cruise conditions may now be inadequate to drive certain of the vehicle's vacuum powered accessories. Further, provision of an extensive network of limited access highways throughout the United States has made it possible to drive a vehicle under steady state cruising conditions for several hours at a time, yielding the possibility that even a substantial vacuum storage volume may not be sufficient to maintain adequate vacuum for operating certain vehicle accessories during the period in which constant operation at normal cruising speed may be possible.